1. Field of the Invention
The invention relates to orthopedic joint replacement and, more particularly, to a prosthetic device for use in orthopedic joint replacement and a method for implanting the prosthetic device.
2. Description of Related Art
FIG. 1A illustrates the bones of a hip joint 10, including a portion of a pelvis 12 and a proximal end of a femur 14. The proximal end of the femur 14 includes a ball-shaped femoral head 16 disposed on a femoral neck 18 that is connected to a femoral shaft 20. The proximal end of the femur 14 has a superior aspect 14a and an inferior aspect 14b. As shown in FIG. 1B, the superior aspect 14a includes a vascular region 24 located near the surface of the bone and having a high concentration of retinacular vessels 24a, which supply blood to the bone tissue of the femoral head 16. As shown in FIG. 1C, the femoral head 16 fits into a concave socket in the pelvis 12 called the acetabulum 22, forming the hip joint 10. The acetabulum 22 and the femoral head 16 are covered by articular cartilage 23 and enclosed within a fibrous joint capsule 21 that is lined with a synovial membrane 25 that secretes synovial fluid 25a. The synovial fluid 25a is a viscous fluid that performs a number of functions vital to joint health, including lubricating the joint 10, delivering nutrients and oxygen to the cartilage 23, removing debris, and inhibiting bacterial growth. Additionally, the synovial fluid 25a and the cartilage 23 work together to absorb shock and reduce friction during articulation of the joint 10.
Over time, the hip joint 10 may degenerate (for example, due to osteoarthritis) resulting in pain and diminished functionality. To reduce pain and restore functionality, a hip replacement procedure (e.g., total hip arthroplasty or hip resurfacing) may be necessary. During hip replacement, a surgeon replaces portions of a patient's hip joint 10 with artificial components. In conventional total hip arthroplasty, the surgeon removes the femoral head 16 and neck 18 (shown in FIG. 2A) and replaces the natural bone with a prosthetic femoral component 26 comprising a head 26a, a neck 26b, and a stem 26c (shown in FIG. 2B). As shown in FIG. 2C, the stem 26c of the femoral component 26 is anchored in a cavity that the surgeon creates in the intramedullary canal of the femur 14. The natural acetabulum 22 of the pelvis 22 may also be replaced. For example, if the acetabulum 22 is worn or diseased, the surgeon can ream the acetabulum 22 and replace the natural surface with a prosthetic acetabular component 28 comprising a hemispherical shaped cup 28a (shown in FIG. 2B) that may include a liner 28b. In cases where the acetabulum 22 is healthy, the surgeon may leave the natural acetabulum 22 intact and replace only the femoral head 16 and neck 18.
In contrast to total hip arthroplasty, which is highly invasive, patients who have healthy subsurface bone and disease that is confined to the surface of the femoral head 16 may be candidates for hip resurfacing. In conventional hip resurfacing, the surgeon removes diseased bone from the femoral head 16 using a rotationally symmetric cutting tool, such as a cylindrical reamer 30. As shown in FIG. 3A, the surgeon centers the cylindrical reamer 30 on an axis A-A defined by a guide hole G created in the femoral head 16. In operation, the cutting element of the cylindrical reamer 30 rotates about the femoral head 16, cutting away diseased surface bone and resulting in a femoral head 16 having a rotationally symmetric surface shape 16a. As shown in FIGS. 3B and 3C, the reamed femoral head is mated with a prosthetic femoral head cup 32. The femoral head cup 32 typically has an internal surface shape that substantially corresponds to the rotationally symmetric surface shape 16a of the reamed femoral head so that the cup 32 will fit securely in place. The femoral head cup 32 also includes a central stem 32a that is received in the guide hole G to aid in alignment and stability of the femoral head cup 32. As with conventional hip arthroplasty, hip resurfacing may include replacement of the acetabulum 22 when the acetabulum 22 is damaged or diseased.
As can be seen by comparing FIGS. 2C and 3B, hip resurfacing is less invasive and preserves more bone than conventional hip arthroplasty because only a portion of the femoral head 16 is removed, leaving the femoral neck 18, the subsurface bone of the femoral head 16, and the intramedullary canal of the femur 14 intact. Although conventional hip resurfacing removes less bone than conventional hip arthroplasty, the procedure still removes a significant portion of the femoral head 16, including healthy bone. As shown in FIG. 3D, one disadvantage of the conventional resurfacing process is that the bone cuts may impinge upon the vascular region 24 of the femur 14 resulting in damage to the retinacular vessels 24a. This damage adversely impacts the blood supply to the femoral head 16, which can ultimately lead to necrosis of the bone, loosening of the implanted femoral head cup 32, pain, and femoral fracture. Additionally, if the cylindrical reamer 30 is undersized or malpositioned, there is a danger of the cylindrical reamer 30 contacting the femoral neck 18, creating a notch in the femoral neck 18. This femoral “notching” causes a stress riser in the femur 14 that increases the risk of femoral fracture, particularly if the notching occurs on the superior aspect 14a of the femoral neck 18, which is in tension during activities such as standing, walking, and running.
Another disadvantage of conventional femoral resurfacing components is that such components may lack the ability to maintain contact with a substantial portion of the articular surface of the acetabular component throughout the range of motion of the joint 10. For example, as the femur 14 moves through the range of motion, the edge of a conventional femoral head cup 32 may articulate above the rim of the acetabular cup thereby reducing the contact area between the femoral and acetabular components. Reduced contact area diminishes the load bearing capability of the component. Additionally, reduced contact area decreases the piston effect, which refers to the vacuum created between the femoral and acetabular components when a force attempts to extract the femoral component from the acetabular component. A strong piston effect creates a high vacuum, which aids in preventing joint dislocation, a painful complication in which the femoral ball disengages from the acetabular cup.
Another disadvantage of conventional hip replacement components is the potential for lever arm dislocation at the extreme range of motion due to impingement, which occurs when the femoral neck impinges on the rim of the acetabular cup creating a lever arm that forces the femoral ball out of the acetabular cup. Impingement is most likely to occur when the hip joint 10 exceeds 90 degrees of flexion, such as when the patient crosses his legs or sits in a low seat where the knees are elevated above the hips. FIGS. 4A to 4D illustrate the mechanics of lever arm dislocation using the total hip prosthesis shown in FIG. 2B. For example, at 90 degrees of flexion (FIG. 4A), the neck 26b of the femoral component 26 is near the rim of the cup-shaped acetabular component 28. If the patient continues to move (flex) the leg beyond 90 degrees (FIG. 4B), the femoral neck 26b contacts or impinges on the rim of the acetabular component 28. As shown in FIG. 4C, the femur 14 acts as a lever mechanism that pushes the femoral neck 26b against the rim of the acetabular component 28, eventually forcing the femoral head 26a out of the acetabular component 28. The resulting dislocation is illustrated in FIG. 4D.
Another disadvantage of conventional hip resurfacing components is that such components may not provide a necessary or desired amount of joint lubrication. The components rely primarily on the clearance between the femoral and acetabular components to draw fluid between the components to achieve joint lubrication. As the patient moves, the clearance angle and motion of the articular surfaces generate a hydrodynamic fluid film that lubricates the joint 10. Factors such as bone density and implant position, however, may impact the ability of the surgeon to achieve optimal clearance. As a result, the ability of the bearing to generate a lubricating fluid film may be compromised.